JP6300948B2 - Polymer piezoelectric film - Google Patents

Description

  The present invention relates to a polymer piezoelectric film.

Conventionally, PZT (PbZrO ₃ —PbTiO ₃ -based solid solution), which is a ceramic material, has been frequently used as the piezoelectric material. However, since PZT contains lead, currently, as a piezoelectric material, a polymer piezoelectric material (polymer piezoelectric film) having a low environmental load and high flexibility has come to be used.
Currently known polymer piezoelectric materials include, for example, nylon 11, polyvinyl fluoride, polyvinyl chloride, polyurea, polyvinylidene fluoride (β-type) (PVDF), vinylidene fluoride-trifluoroethylene copolymer (P Pauling type polymer represented by (VDF-TrFE)) (75/25).

In recent years, attention has been paid to the use of polymers having optical activity, such as polypeptides and polylactic acid, in addition to the above-described polymeric piezoelectric materials. It is known that polylactic acid polymers exhibit piezoelectricity only by mechanical stretching operation.
Among the polymers having optical activity, the piezoelectricity of a polymer crystal such as polylactic acid is attributed to a C = O bond permanent dipole that exists in the direction of the helical axis. In particular, polylactic acid has a small volume fraction of side chains with respect to the main chain and a large ratio of permanent dipoles per volume, and can be said to be an ideal polymer among the polymers having helical chirality. It is known that polylactic acid that exhibits piezoelectricity only by stretching treatment does not require poling treatment and the piezoelectricity does not decrease over several years.

As described above, since polylactic acid has various piezoelectric characteristics, polymer piezoelectric materials using various polylactic acids have been reported. For example, a polymer piezoelectric material obtained by stretching a sheet molded from an aliphatic polyester composition mainly in a uniaxial direction is disclosed (for example, see Document 1). Moreover, the technique which extends | stretches the unstretched film using polylactic acid to a biaxial direction, and makes it a laterally stretched film (biaxially stretched film) is disclosed (for example, refer literature 2 and 3).
Document 1 Japanese Patent Application Laid-Open No. 2014-086703 Document 2 Japanese Patent Application Laid-Open No. 2011-243606 Document 3 Japanese Patent Application Laid-Open No. 2012-153023

  By the way, the polymer piezoelectric film needs to align molecular chains in one direction in order to develop piezoelectricity. For example, in the case of the longitudinally uniaxially stretched film described in Document 1, since the stretching direction (direction in which molecular chains are oriented) is the MD (Machine Direction) direction, the film tends to tear in a direction parallel to the MD direction. There was a problem that the tear strength of the steel was low. Hereinafter, the tear strength in a specific direction is also referred to as “longitudinal tear strength”.

  On the other hand, when a biaxial stretching facility is used, the film can be stretched in both the MD direction and the TD (Transverse Direction) direction orthogonal to the MD direction. For example, the documents 2 and 3 describe a technique relating to a laterally stretched film that mainly stretches in the TD direction when stretching in the biaxial direction. A transversely stretched film is more advantageous from the viewpoint of film production capacity because it is easier to produce a wide film than a longitudinally uniaxially stretched film.

  However, in the case of a laterally stretched film, the longitudinal tear strength in the TD direction is low, and it is easy to tear in a direction parallel to the TD direction. And in the continuous production process of a film, since tension | tensile_strength generate | occur | produces in MD direction, it is easy to produce the fracture | rupture in the TD direction of a film in a production process, and long-term continuous production is difficult. Such production stoppage due to breakage in the TD direction does not occur at the time of production of a longitudinally uniaxially stretched film, and is a major problem in producing a transversely stretched film.

  Moreover, in order to obtain a film having a high longitudinal crack strength, it is generally preferable to form a film by increasing the vertical and horizontal magnifications. The piezoelectricity decreases.

  On the other hand, as a result of intensive studies, the inventors have adjusted the longitudinal and lateral stretching ratios to maintain the piezoelectricity, and further, the elastic modulus in the direction of 45 ° with respect to the stretching direction (MD direction). The present inventors have found that there is a region where the yield stress is increased and the substantial sensor sensitivity is improved when the polymer piezoelectric film is used for a piezoelectric sensor device or the like.

  Therefore, the present invention is to provide a polymer piezoelectric film that can maintain high sensor sensitivity when used in a device, has high longitudinal crack strength, and is excellent in productivity.

Specific means for achieving the above object are as follows.
<1> A helical chiral polymer having an optical activity with a weight average molecular weight of 50,000 to 1,000,000, crystallinity obtained by DSC method of 20% to 80%, and microwave transmission molecular orientation the product of the normalized molecular orientation MORc and the crystallinity when the reference thickness as measured by total and 50μm is from 40 to 700, the refractive index in a slow axis direction in the film plane n _x, film fast-axis refractive index n _y in the plane, the refractive index in the thickness direction of the film and n _z, Nz coefficient = (n _x -n _z) / when the (n _x -n _y), Nz A polymer piezoelectric film having a coefficient in the range of 1.108 to 1.140.

<2> internal haze to visible light is 40% or less, and the stress at 25 ° C. - piezoelectric constant _{d 14} measured by the charge method is 1 pC / N or more, polymeric piezoelectric film according to <1>.

  <3> The polymer piezoelectric film according to <1> or <2>, wherein the internal haze with respect to visible light is 20% or less.

<4> The polymer piezoelectric according to any one of <1> to <3>, wherein the helical chiral polymer is a polymer having a main chain including a repeating unit represented by the following formula (1): the film.

  <5> The polymeric piezoelectric film according to any one of <1> to <4>, wherein the helical chiral polymer has an optical purity of 95.00% ee or more.

  <6> The polymer piezoelectric film according to any one of <1> to <5>, wherein the content of the helical chiral polymer is 80% by mass or more.

<7> the slow axis direction of the refractive indices _{n x} in the film plane is in the range of 1.4720 to 1.4740, polymeric piezoelectric film according to any one of <1> to <6>.

  <8> The polymer piezoelectric film according to any one of <1> to <7>, wherein a piezoelectric constant measured by a stress-charge method is 6 pC / N or more.

  <9> The polymer piezoelectric film according to any one of <1> to <8>, wherein the Nz coefficient is in a range of 1.109 to 1.130.

  <10> The polymer piezoelectric film according to any one of <1> to <9>, wherein an internal haze with respect to visible light is 1% or less.

  <11> It is a biaxially stretched film, the stretch ratio in the direction with a large stretch ratio is the main stretch ratio, the stretch ratio in the direction perpendicular to the direction with the large stretch ratio and parallel to the film surface is the secondary stretch ratio. Sometimes, the polymer piezoelectric film according to any one of <1> to <10>, wherein a main draw ratio / secondary draw ratio is 3.0 to 3.5.

  <12> The polymer piezoelectric film according to <11>, wherein a product of the main draw ratio and the secondary draw ratio is 4.6 to 5.6.

  ADVANTAGE OF THE INVENTION According to this invention, the sensor sensitivity at the time of using for a device can be maintained highly, and the polymeric piezoelectric film which is high in longitudinal crack strength and excellent in productivity can be provided.

Is a graph showing the relationship between the Nz coefficients and _{d 14} × E × _σ in Examples 1 and 2.

  In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.

  Moreover, in this specification, a film surface means the main surface of a film. Here, the “main surface” means the surface having the largest area among the surfaces of the polymer piezoelectric film. The polymer piezoelectric film of the present invention may have two or more main surfaces. For example, when the polymer piezoelectric film has two surfaces A each having a 10 mm × 0.3 mm square surface A, a 3 mm × 0.3 mm square surface B, and a 10 mm × 3 mm square surface C, the polymer piezoelectric film The main surface of the film is surface C, which has two main surfaces.

<Polymer piezoelectric film>
The polymer piezoelectric film of the present invention includes a helical chiral polymer having optical activity having a weight average molecular weight of 50,000 to 1,000,000, the crystallinity obtained by DSC method is 20% to 80%, and The product of the normalized molecular orientation MORc and the crystallinity when the reference thickness measured with a microwave transmission type molecular orientation meter is 50 μm is 40 to 700, and the refraction in the slow axis direction in the film plane rate of n _x, the fast-axis refractive index n _y in the film plane, the refractive index in the thickness direction of the film and n _z, Nz coefficient _{= (n x -n z) /} (n x -n y ), The Nz coefficient is in the range of 1.108 to 1.140.

By using a piezoelectric material with the above configuration, the sensor sensitivity when used in a device can be maintained high, and the longitudinal piezoelectric strength (tear strength in a specific direction) is high, resulting in excellent productivity. It can be a film.
More specifically, when the Nz coefficient is in the range of 1.108 to 1.140, for example, it is possible to maintain a high parameter related to the sensor sensitivity evaluated from the piezoelectric constant d ₁₄ , the elastic modulus, and the yield stress. It is possible to provide a polymer film that can be suitably used for various sensors. Furthermore, since the polymer film of the present invention is excellent in longitudinal crack strength, breakage during production is suppressed and the productivity is excellent.

In the present specification, a decrease in tear strength in a specific direction is sometimes referred to as “longitudinal tear strength decreases”, and a state in which the tear strength in a specific direction is low is referred to as “longitudinal tear strength is low”. Sometimes.
In addition, in this specification, the phenomenon in which the tear strength in a specific direction is suppressed is sometimes referred to as “longitudinal tear strength is improved”, and the phenomenon in which the tear strength in a specific direction is decreased The suppressed state is sometimes referred to as “high longitudinal crack strength” or “excellent longitudinal crack strength”.

[Helical chiral polymer with optical activity]
The helical chiral polymer having optical activity refers to a polymer having molecular optical activity in which the molecular structure is a helical structure.
Hereinafter, the helical chiral polymer having optical activity having a weight average molecular weight of 50,000 to 1,000,000 is also referred to as “optically active polymer”.
Examples of the optically active polymer include polypeptides, cellulose, cellulose derivatives, polylactic acid resins, polypropylene oxide, poly (β-hydroxybutyric acid), and the like. Examples of the polypeptide include poly (glutarate γ-benzyl), poly (glutarate γ-methyl), and the like. Examples of the cellulose derivative include cellulose acetate and cyanoethyl cellulose.

  The optically active polymer preferably has an optical purity of 95.00% ee or more, more preferably 97.00% ee or more, from the viewpoint of improving the piezoelectricity of the polymer piezoelectric film, and 99.00. % Ee or higher is more preferable, and 99.99% ee or higher is particularly preferable. Desirably, it is 100.00% ee. By setting the optical purity of the optically active polymer within the above range, it is considered that the packing property of the polymer crystal that exhibits piezoelectricity is enhanced, and as a result, the piezoelectricity is enhanced.

In the present embodiment, the optical purity of the optically active polymer is a value calculated by the following formula.
Optical purity (% ee) = 100 × | L body weight−D body weight | / (L body weight + D body weight)
That is, “the amount difference (absolute value) between the amount of L-form [mass%] of optically active polymer and the amount of D-form [mass%] of optically active polymer” is expressed as The value obtained by multiplying (multiplying) the value obtained by dividing (dividing) the amount [mass%] by the total amount of the amount [mass%] of the optically active polymer D-form [mass%] and multiplying it by 100. And

  In addition, the value obtained by the method using a high performance liquid chromatography (HPLC) is used for the quantity [mass%] of the L body of an optically active polymer, and the quantity [mass%] of the D body of an optically active polymer. Details of the specific measurement will be described later.

  Among the above optically active polymers, a polymer having a main chain containing a repeating unit represented by the following formula (1) is preferable from the viewpoint of increasing optical purity and improving piezoelectricity.

  Examples of the compound having the repeating unit represented by the formula (1) as a main chain include polylactic acid resins. Among them, polylactic acid is preferable, and L-lactic acid homopolymer (PLLA) or D-lactic acid homopolymer (PDLA) is most preferable.

  The polylactic acid-based resin refers to “polylactic acid”, “copolymer of L-lactic acid or D-lactic acid and a copolymerizable polyfunctional compound”, or a mixture of both. The above-mentioned “polylactic acid” is a polymer in which lactic acid is polymerized by an ester bond and is connected for a long time, a lactide method via lactide, and a direct polymerization method in which lactic acid is heated in a solvent under reduced pressure and polymerized while removing water It is known that it can be manufactured by, for example. Examples of the “polylactic acid” include L-lactic acid homopolymers, D-lactic acid homopolymers, block copolymers containing at least one polymer of L-lactic acid and D-lactic acid, and L-lactic acid and D-lactic acid. Examples include graft copolymers containing at least one polymer.

  Examples of the “copolymerizable polyfunctional compound” include glycolic acid, dimethyl glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 2-hydroxypropanoic acid, 3-hydroxypropanoic acid, 2-hydroxyvaleric acid, 3 -Hydroxyvaleric acid, 4-hydroxyvaleric acid, 5-hydroxyvaleric acid, 2-hydroxycaproic acid, 3-hydroxycaproic acid, 4-hydroxycaproic acid, 5-hydroxycaproic acid, 6-hydroxycaproic acid, 6-hydroxy Hydroxycarboxylic acids such as methyl caproic acid and mandelic acid, glycolides, cyclic esters such as β-methyl-δ-valerolactone, γ-valerolactone, and ε-caprolactone, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid , Pimelic acid, azelaic acid, sebacic acid, undecanedioic acid, Polycarboxylic acids such as candioic acid and terephthalic acid, and anhydrides thereof, ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1 , 4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, tetra Examples thereof include polyhydric alcohols such as methylene glycol and 1,4-hexanedimethanol, polysaccharides such as cellulose, and aminocarboxylic acids such as α-amino acids.

  Examples of the “copolymerizable polyfunctional compound” include the compounds described in paragraph 0028 of International Publication No. 2013/054918 pamphlet.

  Examples of the “copolymer of L-lactic acid or D-lactic acid and a copolymerizable polyfunctional compound” include a block copolymer or a graft copolymer having a polylactic acid sequence capable of forming a helical crystal.

  The concentration of the structure derived from the copolymer component in the optically active polymer is preferably 20 mol% or less. For example, when the optically active polymer is a polylactic acid polymer, the number of moles of the structure derived from lactic acid in the polylactic acid polymer and the structure derived from a compound copolymerizable with lactic acid (copolymer component) It is preferable that the copolymer component is 20 mol% or less based on the total.

The optically active polymer (for example, polylactic acid-based resin) is obtained by, for example, a method obtained by directly dehydrating and condensing lactic acid described in JP-A-59-096123 and JP-A-7-033861. It can be produced by a ring-opening polymerization method using lactide, which is a cyclic dimer of lactic acid, described in Patents 2,668,182 and 4,057,357.
Furthermore, the optically active polymer (for example, polylactic acid-based resin) obtained by each of the above production methods has an optical purity of 95.00% ee or more, for example, when polylactic acid is produced by the lactide method, It is preferable to polymerize lactide whose optical purity is improved to 95.00% ee or higher by crystallization operation.

[Weight average molecular weight of optically active polymer]
The optically active polymer according to this embodiment has a weight average molecular weight (Mw) of 50,000 to 1,000,000. When the lower limit of the weight average molecular weight of the optically active polymer is 50,000 or more, the mechanical strength when the optically active polymer is molded is improved. The lower limit of the weight average molecular weight of the optically active polymer is preferably 100,000 or more, and more preferably 150,000 or more. On the other hand, when the upper limit of the weight average molecular weight of the optically active polymer is 1,000,000 or less, the moldability when a polymer piezoelectric film is obtained by molding (for example, extrusion molding) is improved.
The upper limit of the weight average molecular weight is preferably 800,000 or less, and more preferably 300,000 or less.

  Further, the molecular weight distribution (Mw / Mn) of the optically active polymer is preferably 1.1 to 5, and more preferably 1.2 to 4, from the viewpoint of the strength of the polymer piezoelectric film. Furthermore, it is preferable that it is 1.4-3. The weight average molecular weight Mw and molecular weight distribution (Mw / Mn) of the polylactic acid polymer are measured by gel permeation chromatography (GPC) by the following GPC measurement method.

-GPC measuring device-
Waters GPC-100
-Column-
Made by Showa Denko KK, Shodex LF-804
-Sample preparation-
A polylactic acid polymer is dissolved in a solvent (for example, chloroform) at 40 ° C. to prepare a sample solution having a concentration of 1 mg / mL.
-Measurement conditions-
0.1 mL of the sample solution is introduced into the column at a solvent [chloroform], a temperature of 40 ° C., and a flow rate of 1 mL / min.

  The sample concentration in the sample solution separated by the column is measured with a differential refractometer. A universal calibration curve is created with a polystyrene standard sample, and the weight average molecular weight (Mw) and molecular weight distribution (Mw / Mn) of the polylactic acid polymer are calculated.

Commercially available polylactic acid may be used as the polylactic acid polymer. Commercially available polylactic acid, for example, PURAC Co. PURASORB (PD, PL), manufactured by Mitsui Chemicals, Inc. of LACEA (H-100, H- 400), NatureWorks LLC Corp. ^Ingeo TM Biopolymer, and the like.

  When a polylactic acid resin is used as the optically active polymer, the optically active polymer should be produced by the lactide method or the direct polymerization method in order to increase the weight average molecular weight (Mw) of the polylactic acid resin to 50,000 or more. Is preferred.

The polymer piezoelectric film according to this embodiment may contain only one kind of the optically active polymer described above, or may contain two or more kinds.
In the polymer piezoelectric film according to the present embodiment, the content of the optically active polymer (helical chiral polymer) (the total content in the case of two or more types; the same shall apply hereinafter) is not particularly limited. It is preferable that it is 80 mass% or more with respect to the molecular piezoelectric film total mass.
When the content is 80% by mass or more, the piezoelectric constant tends to be larger.

(Stabilizer)
The polymeric piezoelectric film according to the present embodiment contains a compound having a weight average molecular weight of 200 to 60000 having at least one functional group selected from the group consisting of a carbodiimide group, an epoxy group, and an isocyanate group as a stabilizer. May be.
Thereby, the hydrolysis reaction of the optically active polymer (helical chiral polymer) can be suppressed, and the moisture and heat resistance of the resulting film can be further improved.
Regarding the stabilizer, the description in paragraphs 0039 to 0055 of International Publication No. 2013/054918 pamphlet can be appropriately referred to.

(Antioxidant)
Moreover, the polymer piezoelectric film according to the present embodiment may contain an antioxidant. The antioxidant is at least one compound selected from a hindered phenol compound, a hindered amine compound, a phosphite compound, and a thioether compound.
Moreover, it is preferable to use a hindered phenol compound or a hindered amine compound as the antioxidant. Thereby, the polymeric piezoelectric film which is excellent also in heat-and-moisture resistance and transparency can be provided.

(Other ingredients)
The polymer piezoelectric film according to the present embodiment is a known resin typified by polyvinylidene fluoride, polyethylene resin and polystyrene resin, inorganic fillers such as silica, hydroxyapatite, and montmorillonite, as long as the effects of the present invention are not impaired. It may contain other components such as a known crystal nucleating agent such as phthalocyanine.
When the polymer piezoelectric film contains a component other than the helical chiral polymer, the content of the component other than the helical chiral polymer is preferably 20% by mass or less based on the total mass of the polymer piezoelectric film, More preferably, it is 10 mass% or less.

  The polymer piezoelectric film of the present embodiment is an optically active polymer as described above (that is, a helical having an optical activity having a weight average molecular weight (Mw) of 50,000 to 1,000,000) as long as the effects of the present embodiment are not impaired. A helical chiral polymer other than (chiral polymer) may be included.

-Inorganic filler-
The polymer piezoelectric film of the present embodiment may contain at least one inorganic filler.
For example, an inorganic filler such as hydroxyapatite may be nano-dispersed in the polymer piezoelectric film in order to make the polymer piezoelectric film a transparent film in which voids such as bubbles are suppressed. In order to make nano-dispersion, a large energy is required for crushing the aggregate, and when the inorganic filler is not nano-dispersed, the transparency of the film may be lowered. Therefore, when the polymer piezoelectric film according to this embodiment contains an inorganic filler, the content of the inorganic filler with respect to the total mass of the polymer piezoelectric film is preferably less than 1% by mass.
When the polymer piezoelectric film contains components other than the optically active polymer, the content of components other than the optically active polymer is preferably 20% by mass or less based on the total mass of the polymer piezoelectric film, More preferably, it is 10 mass% or less.

-Crystal accelerator (crystal nucleating agent)-
The polymer piezoelectric film of the present embodiment may contain at least one crystallization accelerator (crystal nucleating agent).
The crystal accelerator (crystal nucleator) is not particularly limited as long as the effect of promoting crystallization is recognized, but is a substance having a crystal structure having a face spacing close to the face spacing of the crystal lattice of the optically active polymer. It is desirable to select. This is because a substance with a close spacing is more effective as a nucleating agent. For example, when a polylactic acid resin is used as the optically active polymer, the organic substances zinc phenylsulfonate, melamine polyphosphate, melamine cyanurate, zinc phenylphosphonate, calcium phenylphosphonate, magnesium phenylphosphonate, inorganic Examples thereof include talc and clay. Among them, zinc phenylphosphonate is most preferable because the face spacing is most similar to the face spacing of polylactic acid and provides a good crystal formation promoting effect. In addition, the commercially available crystal accelerator can be used. Specific examples include zinc phenylphosphonate; Eco Promote (manufactured by Nissan Chemical Industries, Ltd.) and the like.

The content of the crystal nucleating agent is usually 0.01 parts by weight to 1.0 parts by weight, preferably 0.01 parts by weight to 0.5 parts by weight with respect to 100 parts by weight of the optically active polymer. From a viewpoint of an effect and a biomass degree maintenance, Most preferably, it is 0.02 mass part-0.2 mass part.
When the content of the crystal nucleating agent is 0.01 parts by mass or more, the effect of promoting crystallization can be more effectively obtained. When the content of the crystal nucleating agent is less than 1.0 part by mass, the crystallization rate can be more easily controlled.

  In addition, it is preferable that a polymeric piezoelectric film does not contain components other than the helical chiral polymer | macromolecule which has optical activity from a transparency viewpoint.

(Refractive index)
Polymeric piezoelectric film according to the present embodiment, the refractive index in a slow axis direction in the film plane n _x, the refractive index n _y in the fast axis in the film plane, the refractive index in the thickness direction of the film n and it is _z, when the Nz coefficient _{= (n x -n z) /} (n x -n y), Nz coefficient is in the range of 1.108 to 1.140.
In this specification, n _x is the refractive index in a slow axis direction of the main surface of the film at a wavelength of 589nm, n _y is the fast axis direction refractive index in the main surface of the film at a wavelength of 589nm in and, n _z is the refractive index in the thickness direction of the film at a wavelength of 589 nm.

  The refractive index of the polymer piezoelectric film may be measured using a commercially available refractometer, for example, a multi-wavelength Abbe refractometer manufactured by ATAGO, DR-M series, or the like.

  The Nz coefficient may be in the range of 1.108 to 1.140, but is preferably in the range of 1.109 to 1.130, and more preferably in the range of 1.110 to 1.120.

Further, _{n x} is the Nz coefficient is not particularly limited as long as the value that can meet the range of 1.108 to 1.140, preferably in the range of 1.4720 to 1.4760, 1 More preferably in the range of 4720 to 1.4740, and still more preferably in the range of 1.4720 to 1.4730.

n _y is the Nz coefficient is not particularly limited as long as the value that can meet the range of 1.108 to 1.140, preferably in the range of 1.4500 to 1.4550, 1.4510 More preferably, it is in the range of ~ 1.4530. Further, _{n z} is the Nz coefficient is not particularly limited as long as the value that can meet the range of 1.108 to 1.140, preferably in the range of 1.4450 to 1.4530, 1 More preferably, it is in the range of more than 4480 and less than 1.4500.

[Crystallinity]
The crystallinity of the polymer piezoelectric film is determined by the DSC method, and the crystallinity of the polymer piezoelectric film of the present embodiment is 20% to 80%, preferably 30% to 70%, preferably 35%. -60% is more preferable. If the crystallinity is within the above range, the piezoelectricity, transparency, and longitudinal tear strength of the polymer piezoelectric film are well balanced, and when the polymer piezoelectric film is stretched, whitening and breakage are unlikely to occur and it is easy to manufacture.
When the degree of crystallinity is 20% or more, the piezoelectricity of the polymer piezoelectric film is maintained high.
Moreover, it can suppress that a longitudinal crack strength and transparency fall because a crystallinity degree is 80% or less.

  In this embodiment, for example, the crystallinity of the polymer piezoelectric film can be adjusted to a range of 20% to 80% by adjusting the crystallization and stretching conditions when the polymer piezoelectric film is manufactured. .

[Standardized molecular orientation MORc]
The normalized molecular orientation MORc is a value determined based on “molecular orientation degree MOR” which is an index indicating the degree of orientation of the helical chiral polymer.
Here, the molecular orientation degree MOR (Molecular Orientation Ratio) is measured by the following microwave measurement method. That is, a polymer piezoelectric film (sample) is placed in a microwave resonant waveguide of a well-known microwave molecular orientation measuring device (also referred to as a microwave transmission type molecular orientation meter) in the microwave traveling direction. Arrange so that the film surface (film surface) is vertical. Then, in a state where the sample is continuously irradiated with the microwave whose vibration direction is biased in one direction, the sample is rotated by 0 to 360 ° in a plane perpendicular to the traveling direction of the microwave, and the microwave transmitted through the sample is transmitted. The degree of molecular orientation MOR is determined by measuring the strength.

The normalized molecular orientation MORc in the present embodiment is a MOR value when the reference thickness tc is 50 μm, and can be obtained by the following equation.
MORc = (tc / t) × (MOR-1) +1
(Tc: reference thickness to be corrected, t: sample thickness)
The normalized molecular orientation MORc can be measured with a known molecular orientation meter such as a microwave molecular orientation meter MOA-2012A or MOA-6000 manufactured by Oji Scientific Instruments Co., Ltd., at a resonance frequency in the vicinity of 4 GHz or 12 GHz.

The polymer piezoelectric film of this embodiment preferably has a normalized molecular orientation MORc of 1.0 to 15.0, more preferably 3.5 to 10.0, and 4.0 to 8.0. More preferably.
If the normalized molecular orientation MORc is 1.0 or more, there are many optically active polymer molecular chains (for example, polylactic acid molecular chains) arranged in the stretching direction, and as a result, the rate of formation of oriented crystals increases. High piezoelectricity can be expressed.
If the normalized molecular orientation MORc is 15.0 or less, a decrease in transparency due to too many molecular chains of the helical chiral polymer that undergoes molecular orientation is suppressed, and as a result, the transparency of the polymer piezoelectric film remains high. Is done. Furthermore, if the normalized molecular orientation MORc is 15.0 or less, the longitudinal crack strength is further improved.

[Product of normalized molecular orientation MORc and crystallinity]
In this embodiment, the product of the crystallinity of the polymer piezoelectric film and the normalized molecular orientation MORc is 40 to 700. By adjusting to this range, the balance between the piezoelectricity and transparency of the polymer piezoelectric film is good, the dimensional stability is high, and the longitudinal tear strength (that is, the tear strength in a specific direction) is reduced. It is suppressed.
The product of the normalized molecular orientation MORc and the crystallinity of the polymer piezoelectric film is preferably 75 to 600, more preferably 100 to 500, still more preferably 125 to 400, and particularly preferably 150 to 300.

  For example, the above product can be adjusted to the above range by adjusting the crystallization and stretching conditions in producing the polymer piezoelectric film.

  Further, the normalized molecular orientation MORc can be controlled by crystallization conditions (for example, heating temperature and heating time) and stretching conditions (for example, stretching temperature and stretching speed) at the time of producing the polymer piezoelectric film.

The normalized molecular orientation MORc can be converted into a birefringence Δn obtained by dividing the retardation amount (retardation) by the thickness of the film.
Specifically, retardation can be measured using RETS100 manufactured by Otsuka Electronics Co., Ltd. MORc and Δn are approximately in a linear proportional relationship, and when Δn is 0, MORc is 1.

[Piezoelectric constant d ₁₄ (stress-charge method)]
Piezoelectricity of polymeric piezoelectric film, for example, can be assessed by measuring the piezoelectric constant d ₁₄ of the piezoelectric polymer film.
Hereinafter, the stress - describing an example of a method for measuring a piezoelectric constant d ₁₄ due to charge method.

  First, the polymer piezoelectric film is cut to 150 mm in a direction formed by 45 ° with respect to the stretching direction (MD direction) and 50 mm in a direction orthogonal to the direction formed by 45 ° to produce a rectangular test piece. Next, set the test piece obtained on the test stand of Showa Vacuum SIP-600, and deposit Al on one surface of the test piece so that the deposition thickness of aluminum (hereinafter referred to as Al) is about 50 nm. To do. Next, vapor deposition is similarly performed on the other side of the test piece, and Al is coated on both sides of the test piece to form an Al conductive layer.

  A test piece of 150 mm x 50 mm with an Al conductive layer formed on both sides is 120 mm in a direction of 45 ° with respect to the stretching direction (MD direction) of the polymer piezoelectric film, and 10 mm in a direction perpendicular to the direction of 45 °. Cut and cut out a rectangular film of 120 mm × 10 mm. This is a piezoelectric constant measurement sample.

The obtained sample is set in a tensile tester (manufactured by AND, TENSILON RTG-1250) with a distance between chucks of 70 mm so as not to be loosened. The force is periodically applied so that the applied force reciprocates between 4N and 9N at a crosshead speed of 5 mm / min. At this time, in order to measure the amount of charge generated in the sample according to the applied force, a capacitor having a capacitance Qm (F) is connected in parallel to the sample, and the voltage V between the terminals of the capacitor Cm (95 nF) is converted into a buffer amplifier. Measure through. The above measurement is performed under a temperature condition of 25 ° C. The generated charge amount Q (C) is calculated as the product of the capacitor capacitance Cm and the terminal voltage Vm. Piezoelectric constant d ₁₄ is calculated by the following equation.
d ₁₄ = (2 × t) / L × Cm · ΔVm / ΔF
t: Sample thickness (m)
L: Distance between chucks (m)
Cm: Capacitor capacity in parallel (F)
ΔVm / ΔF: Ratio of change in voltage between capacitor terminals to change in force

The higher the piezoelectric constant d ₁₄ is, the larger the displacement of the polymer piezoelectric film with respect to the voltage applied to the polymer piezoelectric film, and conversely, the voltage generated with respect to the force applied to the polymer piezoelectric film increases. It is useful as a film.
Specifically, in the polymeric piezoelectric film in the present embodiment, the stress at 25 ° C. - The piezoelectric constant _{d 14} measured by a charge method, and a 1 pC / N or more, preferably at least pC / N, at least 5 pC / N More preferably, 6 pC / N or more is more preferable. The upper limit of the piezoelectric constant d ₁₄ is not particularly limited, from the viewpoint of the balance, such as transparency, preferably less 50pc / N is a polymer piezoelectric film using a helical chiral polymer, more preferably at most 30 pC / N.
From the viewpoint of the balance with similarly transparency is preferably a piezoelectric constant d ₁₄ measured by a resonance method is not more than 15pC / N.

  In the present specification, the “MD direction” is a direction in which the film flows (Machine Direction), and the “TD direction” is a direction perpendicular to the MD direction and parallel to the main surface of the film (Transverse Direction). ).

[Transparency (internal haze)]
The transparency of the polymer piezoelectric film can be evaluated by, for example, visual observation or haze measurement.
In the polymer piezoelectric film of the present embodiment, the internal haze with respect to visible light (hereinafter also simply referred to as “internal haze”) is preferably 40% or less, more preferably 20% or less, and more preferably 10% or less. More preferably, it is more preferably 5% or less, particularly preferably 2.0% or less, and most preferably 1.0% or less.
The lower the internal haze of the polymer piezoelectric film of the present embodiment, the better. However, from the viewpoint of balance with the piezoelectric constant and the like, it is preferably 0.01% to 15%, preferably 0.01% to 10%. % Is more preferable, 0.1% to 5% is further preferable, and 0.1% to 1.0% is particularly preferable.

In the present embodiment, “internal haze” refers to haze excluding haze due to the shape of the outer surface of the polymer piezoelectric film.
Further, the “internal haze” here is a value when measured at 25 ° C. in accordance with JIS-K7105 with respect to the polymer piezoelectric film.

More specifically, the internal haze (hereinafter also referred to as “internal haze H1”) refers to a value measured as follows.
Specifically, first, a haze in the optical path length direction (hereinafter also referred to as “haze H2”) was measured for a cell having an optical path length of 10 mm filled with silicon oil. Next, the polymer piezoelectric film of this embodiment is immersed in the silicon oil of the cell so that the optical path length direction of the cell and the normal direction of the film are parallel, and the optical path of the cell in which the polymer piezoelectric film is immersed The haze in the long direction (hereinafter also referred to as “haze H3”) is measured. Both haze H2 and haze H3 are measured at 25 ° C. according to JIS-K7105.
Based on the measured haze H2 and haze H3, the internal haze H1 is obtained according to the following formula.
Internal haze (H1) = Haze (H3) -Haze (H2)

The measurement of haze H2 and haze H3 can be performed using, for example, a haze measuring machine [manufactured by Tokyo Denshoku Co., Ltd., TC-HIII DPK].
Moreover, as the silicone oil, for example, “Shin-Etsu Silicone (trademark), model number KF-96-100CS” manufactured by Shin-Etsu Chemical Co., Ltd. can be used.

[Tear strength]
The tear strength (longitudinal tear strength) of the polymer piezoelectric film of the present embodiment conforms to the test method “Right-angle tear method” described in “Tear strength of plastic film and sheet” of JIS K 7128-3. Is evaluated based on the measured tear strength.
Here, the crosshead speed of the tensile tester is 200 mm per minute, and the tear strength is calculated from the following equation.
T = F / d
In the above formula, T represents the tear strength (N / mm), F represents the maximum tear load, and d represents the thickness (mm) of the test piece.

(Dimensional stability)
The polymer piezoelectric film preferably has a low dimensional change rate under heating, particularly at the temperature of the environment in which the polymer piezoelectric film is incorporated and used in a device or device such as a speaker or a touch panel described later. This is because if the dimensions of the piezoelectric material change in the environment of use of the device or the like, the position of the wiring or the like connected to the piezoelectric material may be moved to cause malfunction of the device or the like. As will be described later, the dimensional stability of the polymer piezoelectric film is evaluated by the dimensional change rate before and after being treated for 10 minutes at 150 ° C., which is a temperature slightly higher than the usage environment of the device. The dimensional change rate is preferably 10% or less, and more preferably 5% or less.

  Moreover, there is no restriction | limiting in particular in the thickness of the polymeric piezoelectric film of this embodiment, 10 micrometers-400 micrometers are preferable, 20 micrometers-200 micrometers are more preferable, 20 micrometers-100 micrometers are more preferable, 20 micrometers-80 micrometers are especially preferable.

<Method for producing polymer piezoelectric film>
As a method for producing the polymer piezoelectric film of the present invention, the crystallinity can be adjusted to 20% to 80%, and the product of the normalized molecular orientation MORc and the crystallinity can be adjusted to 40 to 700. The method is not particularly limited as long as the Nz coefficient can be adjusted in the range of 1.108 to 1.140.
As this method, for example, crystallization and stretching (any of which may be first) are performed on an amorphous sheet containing the optically active polymer described above, and the crystallization and stretching It is possible to use a method of adjusting each of these conditions.
Here, “crystallization” is a concept including pre-crystallization described later and annealing treatment described later.

  The amorphous sheet refers to a sheet obtained by heating an optically active polymer alone or a mixture containing the optically active polymer to a temperature equal to or higher than the melting point Tm of the optically active polymer and then rapidly cooling it. . An example of the rapid cooling temperature is 50 ° C.

In the method for producing a polymer piezoelectric film of the present invention, the optically active polymer (such as a polylactic acid polymer) is used alone as a raw material for the polymer piezoelectric film (or an amorphous sheet). Alternatively, a mixture of two or more of the optically active polymers described above (such as polylactic acid polymers), or a mixture of at least one of the optically active polymers described above and at least one of the other components. May be used.
The above mixture is preferably a mixture obtained by melt-kneading.
Specifically, for example, when mixing two or more types of optically active polymers, or when mixing other components (for example, the above-described inorganic filler and crystal nucleating agent) with one or more types of optically active polymers, The optically active polymer to be mixed (along with other components as necessary), using a melt kneader (manufactured by Toyo Seiki Seisakusho Co., Ltd., Labo Plast Mixer), under conditions of mixer rotation speed 30 rpm to 70 rpm, 180 ° C. to 250 ° C., By melt-kneading for 5 to 20 minutes, a blend of a plurality of types of optically active polymers or a blend of optically active polymers and other components such as inorganic fillers can be obtained.

  Hereinafter, although the embodiment of the manufacturing method of the polymer piezoelectric film of the present invention will be described, the method of manufacturing the polymer piezoelectric film of the present invention is not limited to the following embodiment.

  The method for producing a polymer piezoelectric film of the present embodiment includes, for example, an amorphous sheet containing an optically active polymer (that is, a helical chiral polymer having an optical activity with a weight average molecular weight of 50,000 to 1,000,000). A first step of obtaining a pre-crystallized sheet by heating and stretching the pre-crystallized sheet in a biaxial direction (for example, a direction different from the stretching direction simultaneously or sequentially while stretching mainly in a uniaxial direction) A second step).

Moreover, as a method for producing the polymer piezoelectric film of the present embodiment, in the second step, the draw ratio in the direction with a large draw ratio is orthogonal to the main draw ratio, the direction in which the draw ratio is large, and parallel to the film surface. When the stretching ratio in one direction is the secondary stretching ratio, it is preferable to stretch in the biaxial direction so that the main stretching ratio / secondary stretching ratio satisfies 3.0 to 3.5.
Thus, the normalized molecular orientation MORc and the crystal when the crystallinity obtained by DSC method is 20% to 80% and the reference thickness measured by a microwave transmission type molecular orientation meter is 50 μm. A polymer piezoelectric film having a product of the degree of conversion of 40 to 700 and an Nz coefficient of 1.108 to 1.140 can be preferably produced.

In general, increasing the force applied to the film during stretching promotes the orientation of the optically active polymer and increases the piezoelectric constant, while crystallization progresses and the crystal size increases, so that the internal haze increases. There is a tendency. In addition, the dimensional change rate tends to increase due to an increase in internal stress. If force is simply applied to the film, crystals that are not oriented like spherulites are formed. A crystal having a low orientation such as a spherulite increases the internal haze, but hardly contributes to an increase in piezoelectric constant.
Therefore, in order to form a film having a high piezoelectric constant and a low internal haze, it is preferable to efficiently form oriented crystals that contribute to the piezoelectric constant with a minute size that does not increase the internal haze.

From the above points, for example, after preparing the pre-crystallized sheet in which the inside of the sheet is pre-crystallized to form fine crystals (microcrystals) before stretching, the pre-crystallized sheet is stretched, thereby The stretching force can be efficiently applied to the polymer portion having low crystallinity between the microcrystals in the crystallized sheet. Thereby, the optically active polymer can be efficiently oriented in the main stretching direction.
Specifically, by stretching the pre-crystallized sheet, finely oriented crystals are formed in the polymer portion having low crystallinity between the microcrystals and at the same time generated by pre-crystallization. The spherulites are broken, and the lamellar crystals constituting the spherulites are oriented in the stretching direction in a daisy chain connected to the tie molecular chain. As a result, a desired value of MORc can be obtained.
For this reason, by extending | stretching the said precrystallized sheet, a sheet | seat with a low internal haze can be obtained, without reducing a piezoelectric constant largely. Furthermore, a polymer piezoelectric film having excellent dimensional stability can be obtained by adjusting the manufacturing conditions.

  However, in the method of stretching the pre-crystallized sheet, the polymer chains in the low-crystallinity portion inside the pre-crystallized sheet are unwound by stretching, and the molecular chains are aligned in the stretch direction. Although the tear strength with respect to the force from is improved, the tear strength with respect to the force from the direction substantially parallel to the stretching direction may be decreased.

In view of the above points, in the present embodiment, the first step of obtaining a pre-crystallized sheet by heating a non-crystalline sheet containing an optically active polymer, and stretching the pre-crystallized sheet in a biaxial direction. And a second step.
In the present embodiment, in the second step (stretching step), when the pre-crystallized sheet is stretched (also referred to as main stretching) in order to enhance piezoelectricity, it intersects with the stretching direction of the main stretching simultaneously or sequentially. Biaxial stretching is performed to stretch the pre-crystallized sheet in the direction (also referred to as secondary stretching). Thereby, since the molecular chain in the sheet can be oriented not only in the direction of the main stretching axis but also in the direction intersecting with the main stretching axis, the normalized molecular orientation MORc and the crystallinity The product can be suitably adjusted to a specific range (specifically, 40 to 700).
As a result, it is possible to increase the longitudinal crack strength while enhancing the piezoelectricity and maintaining the transparency.

In order to control the normalized molecular orientation MORc, it is important to adjust the heating time and heating temperature of the amorphous sheet in the first step, and the stretching speed and stretching temperature of the pre-crystallized sheet in the second step. is there.
As described above, the optically active polymer is a polymer having molecular optical activity in which the molecular structure is a helical structure.
The amorphous sheet containing the optically active polymer may be commercially available, or may be produced by a known film forming means such as extrusion. The amorphous sheet may be a single layer or a multilayer.

[First step (pre-crystallization step)]
The first step in this embodiment is a step of obtaining a pre-crystallized sheet by heating an amorphous sheet containing an optically active polymer.

  Specifically, the treatment through the first step and the second step in the present embodiment is as follows: 1) Heat-treat the amorphous sheet to obtain a pre-crystallized sheet (the first step). The pre-crystallized sheet thus obtained may be set in a stretching apparatus and then stretched (the second process) (off-line processing), or 2) the amorphous sheet is set in a stretching apparatus. The pre-crystallized sheet is heated by a stretching device (above, the first step), and the obtained pre-crystallized sheet is subsequently stretched by the stretching device (the second step). It is good (inline processing).

In the first step, the heating temperature T for pre-crystallizing the amorphous sheet containing the optically active polymer is not particularly limited. However, in terms of enhancing the piezoelectricity and transparency of the produced polymer piezoelectric film. The glass transition temperature Tg of the optically active polymer and the following formula are preferably satisfied, and the crystallinity is preferably set to 1% to 70%.
Tg−40 ° C. ≦ T ≦ Tg + 40 ° C.
(Tg represents the glass transition temperature of the optically active polymer.)

  Here, the glass transition temperature Tg [° C.] of the optically active polymer and the melting point Tm [° C.] of the optically active polymer mentioned above are calculated with respect to the optically active polymer using the above-mentioned differential scanning calorimeter (DSC). Thus, from the melting endothermic curve when the temperature is increased at a temperature rising rate of 10 ° C./min, the glass transition temperature (Tg) obtained as the inflection point of the curve and the temperature confirmed as the peak value of the endothermic reaction ( Tm).

  In the first step, the heat treatment time for pre-crystallization satisfies the desired crystallinity and the normalized molecular orientation MORc of the polymer piezoelectric film after stretching (after the second step) and after stretching If the product with the crystallinity of the polymer piezoelectric film is adjusted to 40 to 700, preferably 75 to 600, more preferably 100 to 500, still more preferably 125 to 400, and particularly preferably 150 to 300. Good. As the heat treatment time becomes longer, the degree of crystallinity after stretching increases, and the normalized molecular orientation MORc after stretching also increases. When the heat treatment time is shortened, the degree of crystallinity after stretching also decreases, and the normalized molecular orientation MORc after stretching also decreases.

  When the degree of crystallization of the pre-crystallized sheet before stretching increases, the sheet becomes harder and a greater stretching stress is applied to the sheet, so that the portion with relatively low crystallinity in the sheet also becomes more oriented, and after stretching It is considered that the normalized molecular orientation MORc also increases. On the contrary, when the crystallinity of the pre-crystallized sheet before stretching becomes low, the sheet becomes soft and the stretching stress becomes more difficult to be applied to the sheet, so that the portion with relatively low crystallinity in the sheet becomes weakly oriented. The normalized molecular orientation MORc after stretching is also considered to be low.

  The heat treatment time varies depending on the kind or amount of the heat treatment temperature, the thickness of the sheet, the molecular weight of the resin constituting the sheet, the additive, and the like. In addition, the substantial heat treatment time for crystallizing the sheet is the preheating that may be performed before the stretching step (second step) described later, and when the amorphous sheet is preheated at a temperature for crystallization, This corresponds to the sum of the preheating time and the heat treatment time in the precrystallization step before preheating.

  The heat treatment time for the amorphous sheet is preferably from 5 seconds to 60 minutes, and more preferably from 1 minute to 30 minutes from the viewpoint of stabilizing the production conditions. For example, when pre-crystallizing an amorphous sheet containing a polylactic acid resin as an optically active polymer, it is heated at 20 ° C. to 170 ° C. for 5 seconds to 60 minutes (preferably 1 minute to 30 minutes). Is preferred.

In this embodiment, in order to efficiently impart piezoelectricity, transparency, and longitudinal crack strength to the stretched sheet, it is preferable to adjust the crystallinity of the pre-crystallized sheet before stretching.
That is, the reason why the piezoelectricity and the like are improved by stretching is that the spherulite is presumed to be in a spherulitic state, stress due to stretching is concentrated in a portion having a relatively high crystallinity in the pre-crystallized sheet, and the spherulites are being destroyed. Orientation improves the piezoelectricity (piezoelectric constant d ₁₄ ), while stretching stress is applied to a portion having a relatively low crystallinity via the spherulite, and the orientation of this relatively low portion is promoted, and the piezoelectricity (piezoelectricity) This is because the constant d ₁₄ ) is considered to be improved.

  The degree of crystallinity of the stretched sheet is set to 20% to 80%, preferably 30% to 70%, more preferably 35% to 60%. Therefore, the crystallinity immediately before stretching of the pre-crystallized sheet is set to be 1% to 70%, preferably 2% to 60%. What is necessary is just to perform the crystallinity degree of a pre-crystallization sheet | seat similarly to the measurement of the crystallinity degree of the polymeric piezoelectric film of this embodiment after extending | stretching.

  The thickness of the pre-crystallized sheet is mainly determined by the thickness of the polymer piezoelectric film to be obtained by stretching in the second step and the stretching ratio, but is preferably 50 μm to 1000 μm, more preferably about 200 μm to 800 μm. It is.

[Second step (stretching step)]
The stretching method in the second step (stretching step) is not particularly limited. Stretching for forming oriented crystals (also referred to as main stretching) and stretching performed in a direction intersecting the direction of stretching. A combined method can be used. By stretching the polymer piezoelectric film, a polymer piezoelectric film having a large principal surface area can also be obtained.

As for the area of the main surface in this embodiment, the area of the main surface of the polymer piezoelectric film is preferably 5 mm ² or more, and more preferably 10 mm ² or more.

By stretching the polymer piezoelectric film mainly in one direction, the molecular chain of polylactic acid polymer contained in the polymer piezoelectric film can be oriented in one direction and aligned with high density, which is higher It is estimated that piezoelectricity can be obtained.
On the other hand, when stretched in only one direction as described above, the molecular chain of the polymer in the sheet is mainly oriented in the stretching direction, so that there is a risk that the longitudinal crack strength due to the force from the direction substantially perpendicular to the stretching direction is lowered. is there.
Therefore, in the stretching process, when stretching for enhancing piezoelectricity (also referred to as main stretching), simultaneously or sequentially, the pre-crystallized sheet is stretched in a direction crossing the main stretching direction (secondary stretching). By performing biaxial stretching (also referred to as stretching), a polymer piezoelectric film having an excellent balance of piezoelectricity, transparency, and longitudinal crack strength can be obtained.
Here, “sequential stretching” refers to a stretching method in which stretching is performed in a uniaxial direction and then stretching in a direction intersecting with the stretching direction.

The biaxial stretching method in the second step is not particularly limited, and a general method can be used. Specifically, roll stretching (stretching in the MD direction) and tenter stretching (stretching in the TD direction). It is preferable to use a combination of these. At this time, from the viewpoint of production efficiency, it is preferable to set the direction in which the stretching ratio is large (for example, the main stretching direction) to the TD direction and the direction in which the stretching ratio is low (for example, the secondary stretching direction) to the MD direction.
Biaxial stretching may be performed simultaneously or sequentially.
Further, when performing sequential stretching, from the viewpoint of suppressing the longitudinal tearing of the film during the second and subsequent stretching, it is preferable to increase the first stretching ratio, from the viewpoint of suppressing the decrease in piezoelectric constant, It is preferable to reduce the stretching ratio to be performed.
As described above, the “MD direction” is a direction in which the film flows, and the “TD direction” is a direction perpendicular to the MD direction and parallel to the main surface of the film.

  The draw ratio can be adjusted so that the product of the crystallinity, MORc, and crystallinity of the polymer piezoelectric film after stretching (or after annealing if an annealing process described later) is within the above-mentioned range. Although not particularly limited, the draw ratio of main stretching is preferably 2 to 8 times, more preferably 3 to 5 times, and even more preferably 3.5 to 4.5 times. The stretching ratio of the secondary stretching is more preferably 1.1 times to 1.4 times, and further preferably 1.1 times to 1.3 times.

  The draw ratio of the main draw relative to the draw ratio of the secondary draw (main draw ratio / secondary draw ratio) is preferably 3.0 to 3.5, and preferably 3.1 to 3.5. More preferred.

  Moreover, the product of the main draw ratio and the secondary draw ratio is preferably 4.6 to 5.6, more preferably 4.6 to 5.3, and 4.6 to 5.0. More preferably it is.

  Also, the stretching speed is not particularly limited, but usually, the main stretching speed and the secondary stretching speed are adjusted according to the magnification. The stretching speed may be set to a speed usually used, and is not particularly limited, but is often adjusted to a speed at which the film does not break during stretching.

  The stretching temperature of the polymer piezoelectric film is a temperature range higher by about 10 ° C. to 20 ° C. than the glass transition temperature of the polymer piezoelectric film when the polymer piezoelectric film is stretched only by a tensile force as in the biaxial stretching method. It is preferable that

When the pre-crystallized sheet is stretched, preheating may be performed immediately before stretching in order to facilitate stretching of the sheet.
This preheating is generally performed in order to soften the sheet before stretching and facilitate stretching, so that the sheet before stretching is crystallized and does not harden the sheet. It is normal.
However, as described above, in the present embodiment, since pre-crystallization is performed before stretching, the pre-heating may be performed together with pre-crystallization. Specifically, preheating and precrystallization can be performed by performing preheating at a temperature higher or longer than the normal temperature in accordance with the heating temperature and heat treatment time in the precrystallization step described above.

[Annealing process]
From the viewpoint of improving the piezoelectric constant, it is preferable that the polymer piezoelectric film after the stretching treatment is subjected to a certain heat treatment (hereinafter also referred to as “annealing treatment”). The temperature of the annealing treatment is preferably about 80 ° C to 160 ° C, and more preferably 100 ° C to 155 ° C.
The temperature application method for the annealing treatment is not particularly limited, and examples thereof include a method of directly heating using a hot air heater or an infrared heater, and a method of immersing the polymer piezoelectric film in a liquid such as heated silicon oil. At this time, when the polymer piezoelectric film is deformed by linear expansion, it is difficult to obtain a practically flat film. Therefore, a certain tensile stress (for example, 0.01 MPa to 100 MPa) is applied to the polymer piezoelectric film, It is preferable to apply temperature while preventing the polymer piezoelectric film from sagging.

  The temperature application time for the annealing treatment is preferably 1 second to 60 minutes, more preferably 1 second to 300 seconds, and even more preferably heating in the range of 1 second to 60 seconds. By annealing for 60 minutes or less, it is possible to suppress a decrease in the degree of orientation due to the growth of spherulites from the molecular chains of the amorphous part at a temperature higher than the glass transition temperature of the polymeric piezoelectric film, As a result, a decrease in piezoelectricity can be suppressed.

The polymer piezoelectric film annealed as described above is preferably quenched after being annealed.
In the annealing treatment, “rapidly cool” means that the annealed polymer piezoelectric film is immersed in ice water or the like immediately after the annealing treatment and cooled to at least the glass transition temperature Tg or less. It means that no other treatment is included in the dipping time.

The rapid cooling method includes a method of immersing the annealed polymer piezoelectric film in a coolant such as ethanol, methanol, or liquid nitrogen containing water, ice water, ethanol, or dry ice, or by spraying a liquid spray with a low vapor pressure to evaporate. The method of cooling by latent heat is mentioned.
In order to continuously cool the polymer piezoelectric film, it is possible to rapidly cool the polymer piezoelectric film by bringing it into contact with a metal roll controlled to a temperature not higher than the glass transition temperature Tg of the polymer piezoelectric film. It is. Further, the number of times of cooling may be only once, or may be two or more. Furthermore, annealing and cooling can be alternately repeated. In addition, when the above-described annealing is performed on the polymer piezoelectric film that has been subjected to the above-described stretching treatment, the polymer piezoelectric film after the annealing may shrink as compared to before the annealing.

<Application of polymer piezoelectric film>
The polymer piezoelectric film of the present invention includes a speaker, a headphone, a touch panel, a remote controller, a microphone, an underwater microphone, an ultrasonic transducer, an ultrasonic applied measuring instrument, a piezoelectric vibrator, a mechanical filter, a piezoelectric transformer, a delay device, a sensor, and an acceleration. Sensor, Shock sensor, Vibration sensor, Pressure sensor, Tactile sensor, Electric field sensor, Sound pressure sensor, Display, Fan, Pump, Variable focus mirror, Sound insulation material, Sound insulation material, Keyboard, Sound device, Information processing machine, Measuring device, The polymer film of the present invention is preferably used particularly in the field of various sensors because it can be used in various fields such as medical equipment and can maintain high sensor sensitivity when used in a device. .
The polymer piezoelectric film of the present invention can also be used as a touch panel combined with a display device. As the display device, for example, a liquid crystal panel, an organic EL panel, or the like can be used.
Moreover, the polymeric piezoelectric film of this invention can also be used in combination with the touch panel (position detection member) of another system as a pressure sensor. Examples of the detection method of the position detection member include an anti-film method, a capacitance method, a surface acoustic wave method, an infrared method, and an optical method.

  At this time, the polymer piezoelectric film of the present invention is preferably used as a piezoelectric element having at least two surfaces and electrodes provided on the surfaces. The electrode may be provided on at least two surfaces of the polymer piezoelectric film. Although it does not restrict | limit especially as said electrode, For example, ITO, ZnO, IZO (trademark), IGZO, a conductive polymer, silver nanowire, a metal mesh etc. are used.

Further, the polymer piezoelectric film of the present invention and an electrode can be repeatedly stacked and used as a laminated piezoelectric element. As an example, a unit in which an electrode and a polymer piezoelectric film are repeatedly stacked, and the main surface of the polymer piezoelectric film that is not covered with an electrode is covered with an electrode. Specifically, the unit having two repetitions is a laminated piezoelectric element in which electrodes, a polymer piezoelectric film, an electrode, a polymer piezoelectric film, and electrodes are stacked in this order. Of the polymer piezoelectric films used in the laminated piezoelectric element, one layer of the polymer piezoelectric film may be the polymer piezoelectric film of the present invention, and the other layers may not be the polymer piezoelectric film of the present invention.
In addition, when the laminated piezoelectric element includes a plurality of polymer piezoelectric films of the present invention, if the optical activity of the optically active polymer contained in one layer of the polymer piezoelectric film of the present invention is L, the other layers The optically active polymer contained in the polymer piezoelectric film may be L-form or D-form. The arrangement of the polymer piezoelectric film can be appropriately adjusted according to the use of the piezoelectric element.

  For example, a first layer of a polymer piezoelectric film containing an L-type optically active polymer as a main component is laminated with a second polymer piezoelectric film containing an L-type optically active polymer as a main component via an electrode. When the uniaxial stretching direction (main stretching direction) of the first polymer piezoelectric film intersects, preferably perpendicularly, the uniaxial stretching direction (main stretching direction) of the second polymer piezoelectric film, This is preferable because the direction of displacement between the polymer piezoelectric film and the second polymer piezoelectric film can be made uniform, and the piezoelectricity of the entire laminated piezoelectric element is enhanced.

  On the other hand, the first layer of the polymer piezoelectric film containing the L-type optically active polymer as the main component is laminated with the second polymer piezoelectric film containing the D-type optically active polymer as the main component via the electrode. When the first polymer piezoelectric film is arranged so that the uniaxial stretching direction (main stretching direction) of the first polymer piezoelectric film is substantially parallel to the uniaxial stretching direction (main stretching direction) of the second polymer piezoelectric film, the first It is preferable because the directions of displacement of the polymer piezoelectric film and the second polymer piezoelectric film can be made uniform, and the piezoelectricity of the entire laminated piezoelectric element is enhanced.

  In particular, when an electrode is provided on the main surface of the polymer piezoelectric film, a transparent electrode is preferably provided. Here, the transparency of the electrode specifically means that the internal haze is 40% or less (total light transmittance is 60% or more).

  The piezoelectric element using the polymer piezoelectric film of the present invention can be applied to the above-described various piezoelectric devices such as speakers and touch panels. In particular, a piezoelectric element provided with a transparent electrode is suitable for application to a speaker, a touch panel, an actuator, and the like.

  Hereinafter, the embodiment of the present invention will be described more specifically by way of examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist of the present invention.

[Example 1]
NatureWorks LLC Inc. polylactic acid resin (product ^name: Ingeo TM Biopolymer, brand: 4032D) were placed in an extruder hopper, extruded from a T-die while heating to 220 to 230 ° C., 0.3 cast rolls 50 ° C. A pre-crystallized sheet having a thickness of 210 μm was formed by contact for a minute (pre-crystallization step). The crystallinity of the pre-crystallized sheet was measured and found to be 4%.
The obtained pre-crystallized sheet was sequentially biaxially stretched to obtain a stretched film (stretching step). Specifically, the pre-crystallized sheet is stretched in the MD direction up to 1.2 times in a roll-to-roll system while heating to 70 ° C. (secondary stretching), and then heated to 75 ° C. with a tenter system. A stretched film was obtained by stretching (main stretching) in the TD direction up to 4.0 times. At this time, the width of the pre-crystallized sheet was 1500 mm, and the feed speed of the pre-crystallized sheet was 5 m / min.
While the film after the stretching step was fixed with a tenter, it was annealed by passing it through a furnace heated to 150 ° C. for 15 seconds and rapidly cooled to produce a polymer piezoelectric film (annealing step). The rapid cooling is performed by rapidly lowering the film temperature to near room temperature by bringing the annealed film into contact with the air at 20 ° C. to 30 ° C. and then contacting with the metal roll of the film winder. It was.

  The physical property values of the polylactic acid resins used in Example 1, Example 2 and Comparative Examples 1 and 2 are as shown in Table 1 below.

[Example 2]
A polymeric piezoelectric film of Example 2 was produced in the same manner as in the production of the polymeric piezoelectric film of Example 1, except that the stretching conditions were changed to the conditions shown in Table 2 below.

[Comparative Examples 1 and 2]
Next, in the production of the polymer piezoelectric film of Example 1, the polymer piezoelectric films of Comparative Examples 1 and 2 were produced in the same manner except that the stretching conditions were changed to the conditions shown in Table 2 below.

-Measurement of L and D amounts of resin (optically active polymer)-
In a 50 mL Erlenmeyer flask, 1.0 g of a sample (polymer piezoelectric film) was weighed, and 2.5 mL of IPA (isopropyl alcohol) and 5 mL of a 5.0 mol / L sodium hydroxide solution were added. Next, the Erlenmeyer flask containing the sample solution was placed in a water bath at a temperature of 40 ° C. and stirred for about 5 hours until the polylactic acid was completely hydrolyzed.

The sample solution was cooled to room temperature, neutralized by adding 20 mL of 1.0 mol / L hydrochloric acid solution, and the Erlenmeyer flask was sealed and mixed well. 1.0 mL of the sample solution was placed in a 25 mL volumetric flask, and HPLC sample solution 1 was prepared with 25 mL of mobile phase. 5 μL of the HPLC sample solution 1 was injected into the HPLC apparatus, the D / L body peak area of polylactic acid was determined under the following HPLC conditions, and the amount of L body and the amount of D body were calculated.
-HPLC measurement conditions-
・ Column Optical resolution column, Sumika Rial OA5000 manufactured by Sumika Chemical Analysis Co.
・ Measuring device: JASCO Corporation liquid chromatography column temperature: 25 ℃
Mobile phase 1.0 mM-copper (II) sulfate buffer / IPA = 98/2 (V / V)
Copper (II) sulfate / IPA / water = 156.4 mg / 20 mL / 980 mL
・ Mobile phase flow rate 1.0mL / min ・ Detector UV detector (UV254nm)

<Molecular weight distribution>
Using a gel permeation chromatograph (GPC), the molecular weight distribution (Mw / Mn) of the resin (optically active polymer) contained in each polymer piezoelectric film of Examples and Comparative Examples was measured by the following GPC measurement method.
-GPC measurement method-
・ Measurement device Waters GPC-100
・ Column Showa Denko KK, Shodex LF-804
-Preparation of sample Each polymeric piezoelectric film of an Example and a comparative example was dissolved in the solvent [chloroform] at 40 degreeC, respectively, and the sample solution with a density | concentration of 1 mg / mL was prepared.
Measurement conditions 0.1 mL of the sample solution was introduced into the column at a solvent (chloroform), a temperature of 40 ° C. and a flow rate of 1 mL / min, and the sample concentration in the sample solution separated by the column was measured with a differential refractometer. For the molecular weight of the resin, a universal calibration curve was prepared using a polystyrene standard sample, and the weight average molecular weight (Mw) of each resin was calculated.

<Measurement and evaluation of physical properties>
For the polymeric piezoelectric films of Examples 1 and 2 and Comparative Examples 1 and 2 obtained as described above, melting point Tm, crystallinity, thickness, internal haze, piezoelectric constant, MORc, dimensional change rate, The tear strength and elongation at break were measured.
The evaluation results are shown in Table 3.
Specifically, the measurement was performed as follows.

[Melting point Tm and crystallinity]
Each of the polymer piezoelectric films of Examples and Comparative Examples was accurately weighed 10 mg each, and measured using a differential scanning calorimeter (DSC-1 manufactured by Perkin Elmer Co., Ltd.) at a temperature rising rate of 10 ° C./min. A melting endotherm curve was obtained. A melting point Tm and a crystallinity were obtained from the obtained melting endotherm curve.

[Dimensional change rate]
Each of the polymer piezoelectric films of Examples and Comparative Examples was cut 50 mm in the MD direction and 50 mm in the TD direction to cut out a 50 mm × 50 mm rectangular film. This film was suspended in an oven set at 100 ° C. and annealed for 30 minutes (hereinafter, this annealing treatment for evaluating the dimensional change rate was referred to as “anneal B”). Thereafter, the dimensions of the film rectangular side length in the MD direction and TD direction before and after annealing B were measured with a two-dimensional measuring machine CRYSTAL μV606 manufactured by Mitutoyo Corporation, and the dimensional change rate (%) was calculated according to the following formula, and the absolute value thereof: Thus, the dimensional stability was evaluated. A smaller dimensional change rate indicates higher dimensional stability.
MD dimensional change rate (%) = 100 × ((side length in MD direction before annealing B) − (side length in MD direction after annealing B)) / (side length in MD direction before annealing B)
TD dimensional change rate (%) = 100 × ((side length in TD direction before annealing B) − (side length in TD direction after annealing B)) / (side length in TD direction before annealing B)

[Internal haze]
The “internal haze” in the present application refers to the internal haze of the polymer piezoelectric film of the present invention, and is measured by the following method.
Specifically, the internal haze (hereinafter also referred to as internal haze (H1)) of each of the polymer piezoelectric films of Examples and Comparative Examples was measured by measuring light transmittance in the thickness direction. More specifically, haze (H2) is measured in advance by sandwiching only silicon oil (Shin-Etsu Silicone (trademark) manufactured by Shin-Etsu Chemical Co., Ltd., model number: KF96-100CS) between two glass plates. The film (polymer piezoelectric film) whose surface was uniformly coated with was sandwiched between two glass plates, the haze (H3) was measured, and by taking these differences as in the following formula, The internal haze (H1) of each polymer piezoelectric film was obtained.
Internal haze (H1) = Haze (H3) -Haze (H2)

The haze (H2) and haze (H3) in the above formula were measured by measuring the light transmittance in the thickness direction using the following apparatus under the following measurement conditions.
Measuring device: manufactured by Tokyo Denshoku Co., Ltd., HAZE METER TC-HIIIDPK
Sample size: 30 mm wide x 30 mm long (see Table 3 for thickness)
Measurement conditions: Conforms to JIS-K7105 Measurement temperature: Room temperature (25 ° C.)

[Piezoelectric constant d ₁₄ (stress-charge method)]
The piezoelectric constant of the crystallized polymer film (specifically, the piezoelectric constant d ₁₄ (stress-charge method)) was measured according to the above-described “Example of Method for Measuring Piezoelectric Constant d ₁₄ by Stress-Charge Method”.

[Standardized molecular orientation MORc]
About each polymeric piezoelectric film of an Example and a comparative example, normalization molecular orientation MORc was measured by Oji Scientific Instruments Co., Ltd. microwave system molecular orientation meter MOA-6000. The reference thickness tc was set to 50 μm.

[Tear strength]
About each polymeric piezoelectric film of an Example and a comparative example, based on the test method "right-angled tear method" described in "Tear strength of a plastic film and a sheet | seat" of JISK7128-3, tear strength of TD direction The thickness (longitudinal crack strength) was measured.
A large tear strength in the TD direction means that a decrease in longitudinal crack strength is suppressed. In other words, the fact that at least one of the tear strengths in the TD direction is low means that the longitudinal tear strength has decreased.
In the measurement of tear strength, the crosshead speed of the tensile tester was 200 mm per minute.
The tear strength (T) was calculated from the following equation.
T = F / d
In the above formula, T represents the tear strength (N / mm), F represents the maximum tear load, and d represents the thickness (mm) of the test piece.

[Elastic modulus, yield stress, elongation at break]
About the rectangular test piece obtained by cutting each polymer piezoelectric film of the example and the comparative example to 180 mm in a direction forming 45 ° with respect to the stretching direction (MD direction) and 10 mm in a direction orthogonal to the direction forming 45 °, Based on JIS-K-7127, a tensile tester made by Toyo Seiki Seisakusho Strograph VD1E was used to measure the elastic modulus, yield stress, and elongation at break in the 45 ° direction.

As shown in Table 3, Examples 1 and 2 showed higher longitudinal crack strength and elongation at break than Comparative Example 1.
In Examples 1 and 2, a higher piezoelectric constant was shown than in Comparative Example 2.

(Refractive index)
The obtained polymer piezoelectric film, the refractive indices _n x at 23 ° _C., n y, and _{n z,} ATAGO Co. multiwavelength Abbe refractometer was determined using a DR-M2. And Nz coefficient was computed based on the following formula | equation.
Nz factor _{= (n x -n z) /} (n x -n y)
Incidentally, n _x is the refractive index in a slow axis direction of the main surface of the film at a wavelength of 589 nm, n _y is a refractive index in a fast axis direction in the main surface of the film at a wavelength of 589 nm, n _z Is the refractive index in the thickness direction of the film at a wavelength of 589 nm.

Table 4 below shows the results of parameters (piezoelectric constant d ₁₄ , elastic modulus E, yield stress σ, d ₁₄ × E × σ) related to the refractive index and sensor sensitivity obtained as described above. The relationship between the Nz coefficient and d ₁₄ × E × σ is shown in FIG.

As shown in Table 2 and 4, in Comparative Example 1, since the vertical (MD) ratio is lower than the first and second embodiments, one value of the piezoelectric constant d ₁₄ is large, the value of the elastic modulus E and yield stress σ It is getting smaller. In Comparative Example 2, since the vertical (MD) ratio is higher than in Examples 1 and 2, and the surface distribution of the orientation deteriorated, while the value of the piezoelectric constant d ₁₄ is smaller, the elastic modulus E and the value of yield stress σ Is getting bigger. In Examples 1 and 2, the values of the piezoelectric constant d ₁₄ , the elastic modulus E, and the yield stress σ can be increased, and the value of d ₁₄ × E × σ, which is a comprehensive parameter of sensor sensitivity, can be maintained large. it can.
In other words, in Examples 1 and 2 in which the Nz coefficient is in the range of 1.108 to 1.140, compared to Comparative Examples 1 and 2 in which the Nz coefficient is outside this range, the value of d ₁₄ × E × σ is In addition, in Examples 1 and 2, the piezoelectric constant could be maintained higher than that in Comparative Example 2. Therefore, the polymer piezoelectric films of Examples 1 and 2 are expected to maintain high sensor sensitivity when used in a device.

  Furthermore, as shown in Table 3, in Comparative Example 1, since the value of the longitudinal crack strength is low, it is likely that line breakage during production is likely to occur and the productivity is low. On the other hand, in Examples 1 and 2, the value of the longitudinal crack strength is high, and it is considered that the line breakage during production hardly occurs and the productivity is excellent.

The disclosure of Japanese Patent Application No. 2014-218539 filed on October 27, 2014 is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards mentioned in this specification are to the same extent as if each individual document, patent application, and technical standard were specifically and individually stated to be incorporated by reference, Incorporated herein by reference.

Claims (10)

Including a helical chiral polymer having an optical activity of weight average molecular weight of 50,000 to 1,000,000, crystallinity obtained by DSC method is 20% to 80%, and measured with a microwave transmission type molecular orientation meter The product of the normalized molecular orientation MORc and the crystallinity when the reference thickness is 50 μm is 40 to 700,
The refractive indices n _x in the slow axis direction of the film plane, the fast-axis refractive index n _y in the film plane, the refractive index in the thickness direction of the film and n _z, Nz coefficient = (n _x - when the _{n z) / (n x -n} y), Ri range near the Nz coefficient is 1.108 to 1.140,
It is a biaxially stretched film, when the draw ratio in the direction where the draw ratio is large is the main draw ratio, when the draw ratio in the direction perpendicular to the direction where the draw ratio is large and parallel to the film surface is the secondary draw ratio, a main draw ratio / secondary draw ratio is 3.0 to 3.5, the product of the main draw ratio and the secondary draw ratio Ru der 4.6 to 5.6, a polymer piezoelectric film . Internal haze to visible light is not less than 40% and stress at 25 ° C. - piezoelectric constant d ₁₄ measured by the charge method is 1 pC / N or more, polymeric piezoelectric film according to claim 1.   The polymer piezoelectric film according to claim 1 or 2, wherein an internal haze with respect to visible light is 20% or less. The polymer piezoelectric film according to any one of claims 1 to 3, wherein the helical chiral polymer is a polymer having a main chain including a repeating unit represented by the following formula (1).
  The polymeric piezoelectric film according to any one of claims 1 to 4, wherein the helical chiral polymer has an optical purity of 95.00% ee or more.   The polymer piezoelectric film according to claim 1, wherein the content of the helical chiral polymer is 80% by mass or more. A slow axis direction of the refractive indices _{n x} in the film plane is in the range of 1.4720 to 1.4740, polymeric piezoelectric film according to any one of claims 1 to 6.   The polymer piezoelectric film according to any one of claims 1 to 7, wherein a piezoelectric constant measured by a stress-charge method is 6 pC / N or more.   The polymer piezoelectric film according to any one of claims 1 to 8, wherein the Nz coefficient is in a range of 1.109 to 1.130.   The polymer piezoelectric film according to any one of claims 1 to 9, wherein an internal haze with respect to visible light is 1% or less.